Fertilized eggs were transferred reciprocally between large and small mothers, to study maternal effects on body weight in mice selected for large and small size, respectively. Prenatal maternal effects were not important in our material, but postnatal maternal effects were detectable. The postnatal effects accrued mostly from the inadequacy of small mothers for large offspring; small offspring were largely unaffected by the type of mother. Genetic maternal effects were only of limited importance; maternal influences of environmental origin arose from variation in litter size.

The types of equilibria possible with an inversion in a two-locus system are considered, and their stability properties investigated. With complete suppression of crossing-over in inversion heterozygotes, there are three possible types of stable equilibria; which of these is reached by a new inversion depends on the fitness effects of the two loci concerned. With one of these equilibria, basically involving cumulative overdominance of the selected loci, the inverted and standard sequences are genetically homogeneous and differ with respect to both loci. With the other types of equilibrium, the standard sequence remains heterogeneous for one or both loci. It is shown that this situation may lead to variations in karyotypic fitnesses when the inversion is changing in frequency. It is also found that, with certain fitness relationships, two alternative stable equilibria may coexist; the final frequency reached by an inversion may therefore depend on the population's history.

The effects of double crossing-over in inversion heterozygotes were also investigated, and it was shown that the equilibria with double crossing-over are closely related to the corresponding equilibria without it, except that both sequences are more heterogeneous genetically. Within each sequence there is almost complete linkage equilibrium between the selected loci, although both are in linkage disequilibrium with the inversion itself. It was also found that, with double crossing-over, the population tends to remain for many thousands of generations in a state of quasi-equilibrium. In this state, the inversion tends not to return to its original frequency after a perturbation; also, it may remain for a long time relatively homogeneous genetically, especially when rare.

These results were compared with those from experiments and observations on inversion polymorphisms.

Heterokaryons of N. crassa were synthesized from homokaryotic strains differing in sterol composition and sensitivity to the polyene antibiotic nystatin. Mycelia of the nystatin-sensitive strain erg-1+ contained ergosterol and episterol, and the nystatin-resistant mutant erg-1 contained fecosterol and lichesterol. Mycelia of heterokaryons with different proportions of erg-1+: erg-1 nuclei contained various proportions of the four sterols. Ergosterol was the principal sterol in heterokaryons with more than 5% erg-1+ nuclei.

Heterokaryons with various proportions of erg-1+:erg-1 nuclei were grown for several weeks along tubes of synthetic media. Growth rates were stable on minimal medium and nutritionally supplemented media but nuclear proportions often fluctuated. Growth rates fell sharply on nystatin-supplemented media and there were adaptive increases in proportions of mutant erg-1 nuclei which resulted in selection of nystatin-resistant homokaryotic mycelia.

Population genetics of extranuclear genomes is further developed under the neutral-mutation random-drift hypothesis, and the characteristic evolutionary aspects are summarized. Several formulae derived here are concerned with the variances of genetic variability (gene identity) at a single extranuclear locus and the evolutionary distance between two isolated populations which is estimated from a comparison of homologous linked nucleotide sites. Two types of variance are considered; one is the variance in the entire population (VQ) and the other is the variance within a single germ cell (VH). When compared with a Mendelian genetic system in a panmictic population, an extranuclear genetic system has the following equilibrium properties: (1) the mean genetic variability is low if, despite the high multiplicity of the genome in a cell, the proportion of the cytoplasmic contribution from the male's gamete is small, (2) the effect of recombination is small and a large amount of variance of linkage disequilibrium tends to be maintained, (3) the overall relationship between the mean and variance of genetic variability does not much differ but VQ (VH) is expected to be small if the paternal contribution is small, and (4) the evolutionary distance estimated depends on the extent of intrapopulational variation in a common ancestor population which in turn depends on within-cell variation. I argue that there is an analogy between the model of extranuclear genomes in a finite population and that of nuclear genes in a subdivided population. The analogy helps our understanding of some properties in an extranuclear genetic system.

Two mouse populations, randombred albino mice and a cross of four inbred strains, were divergently selected for high (H8) and low (L8) 8-week body weight over 18 generations using within-family and individual selection. The crossbreds showed asymmetry of selection response and realized heritabilities (H8 0·29 ± 0·01; L8 0·17 ± 0·01). In the randombred population realized heritabilities were symmetrical (H8 0·23 ± 0·01; L8 0·22 ± 0·02). Over the first nine generations individual selection was nearly 40 per cent better than within-family selection, as was expected from the full sib correlation in both populations. As selection progressed, within-family selection reached 82% and 61% of the responses obtained with individual selection in the crossbreds and randombred respectively. Correlated responses for 3-week (weaning) and 5-week body weights agreed with observations made on direct responses, but selection for L8 did not reduce weaning weight. Selection for L8 decreased and selection for H8 increased first litter size at birth. However, mass-selected L8-pairs had a higher life-reproduction and life-span than H8-pairs.

For 29 generations, populations of Drosophila melanogaster were offered one favourable (standard) and one suboptimal (salt-supplemented) medium, either singly or simultaneously. Egg-to-adult viability, fecundity and choice of oviposition medium were measured at regular intervals on both resources up to 17 generations after initiation of the salt treatment. Except for a decrease in viability on salt medium in the single-resource populations (SRPs) maintained on the optimal medium, these fitness components remained unchanged. Estimation of a more inclusive measure of fitness, productivity, obtained at generations 27–29, revealed that: (1) the SRPs maintained on salt medium were more adapted to salt medium; (2) the mixed-resource populations (MRPs) were intermediate in their adaptation to salt medium between either type of single-resource population. These results support Levins' model of optimal strategy for populations living in a coarse-grained environment when the fitness set is convex. Family selection for increased and decreased resistance to salt in the medium, carried out for the viability component at generations nine and 19, showed that: (1) genetic variation with respect to this component was present in all populations; (2) the SRPs maintained on salt medium had responded to the salt treatment by eliminating sensitive genotypes; (3) in the first selection experiment, the MRPs had a greater amount of additive genetic variance with respect to viability than either type of SRP; in the second experiment, this difference was not significant, but it was in the predicted direction. The latter finding provides some evidence in favour of the hypothesis repeatedly presented in the literature that environmental heterogeneity could promote the maintenance of genetic variability in populations.

A Statistical analysis of the genome of D. melanogaster indicates that functionally related genes tend to be found on a particular chromosome and, when their locations within a given chromosome are measured in terms of map units, show a tendency to cluster; this clustering within chromosomes, however, is completely accounted for by the known clustering of all genes within chromosomes. Thus the analysis does not reveal any obvious functional significance for the arrangement of the genes within the chromosomes of this organism.

Gene and genotypic frequencies for a deleterious mutant in mutation selection balance are derived for an infinite population undergoing partial self-fertilization. These provide formulations of mean survival and the mutational load. Obtained also are the average number of mutant genes and affected individuals stemming from a single mutant.

As a concomitant effect on frequencies at a neutral locus the mutational load is distributed disproportionately among the neutral genotypes. For partially recessive mutant genes on the 1, 1-sh, 1-s scale, the effect is to increase the frequency of the heterozygote and to decrease the frequencies of homozygotes at the neutral locus relative to the frequencies expected with complete neutrality. This apparent overdominance at the neutral locus has been shown to be connected with identity disequilibrium rather than linkage disequilibrium. It increases generally as s and h decrease, and as the proportion of self-fertilization and the degree of linkage increase. The apparent overdominance with complete linkage is generally less than double that for free recombination. For partially dominant mutant genes, h ≥ ½, the effects on the frequencies of heterozygote and homozygotes at the neutral locus are reversed.

1. It has been shown that metagons, gene-controlled units necessary for maintenance of mu particles in Paramecium aurelia, can be extracted from cells containing one or both of the dominant genes M1 and M2, and reinfected into other paramecia. Provided the latter contain mu particles before infection, the introduced metagons resume their normal function of maintaining the mu particles.

2. Homogenates containing metagons have been fractionated in various ways. The metagon activity is found in the sediment after centrifuging at 105,000 g. for 90 min., and is retained after deoxycholate treatment removing the microsomal membranes, and after phenol extraction removing the ribosomal protein. By lowering the magnesium concentration, some metagon activity passes from the microsomes into the supernatant.

3. Metagon activity in RNA fractions is destroyed by ribonuclease at a concentration of 5 μg. per ml.

4. The metagons both in extracts and inside the cells appear often to be aggregated in clumps, the individual units of which are each functionally active in maintaining mu particles in living cells.

5. It is concluded that the metagon may be considered as a very stable and repeatedly functioning form of ‘messenger’ RNA.

Esterase activity was detected in starch-gel electropherograms of plasma samples from two species of teleost: flounder and plaice. In both species polymorphism was found. In flounder its genetic origin was indicated by population genetical data. Differences in staining intensity of the individual bands were found to be correlated with length of the animals.

Amoebae and plasmodia are alternate vegetative forms in the life cycle of the acellular slime mould Physarum polycephalum. Haploid amoebae carrying heterothallic alleles of the matA (or mt) locus ordinarily form plasmodia only by crossing, but occasionally give rise to mutants that form plasmodia by selfing as well as by crossing. Twelve independently isolated mutants of this type have been studied. Eight carry mutations (termed gad or greater asexual differentiation mutations) within approximately 0·2 map units of matA. Another mutation (gad-12) is linked neither to matA nor to any of 9 other markers tested. The remaining three mutations are linked to matA and map as follows: matA–0·5 units – gad-4–4 units – gad-6 – 8 units – gad-11. One mutation, gad-11, has been tested in strains carrying each of the five matA alleles (matAl, 2, 3, 4, and h) available in a common genetic background; the mutation is expressed with all five alleles. The mutation npfF1 (formerly aptA1), which was isolated as a suppressor of selfing in Colonia (matAh) amoebae, suppresses the action of each of the 12 gad mutations. The similarly isolated mutation npfA1 is also epistatic to eight of the mutations, but permits selfing with gad-5, 6, 12 and 13. For double mutant strains containing gad-12 and gad-1, 2, 4, 6 or 11, the selfing behaviour of each double mutant differs from that of either single mutant. Mixtures of gad−npfF1 with gad+npf+ amoebae readily form plasmodia, a result suggesting that gad mutations are dominant or semi-dominant. We conclude that the commitment of a cell to differentiate into a plasmodium is under the control of a complex group of genes linked to matA.

The amount of linkage of disequilibrium maintained in a two-locus infinite population model by gene conversion and recombination is examined. Intrachromosomal conversion (conversion between different loci on the same chromosome) generates positive linkage disequilibrium. Specifically, = p(1 − p) [1 − r/(γ + r − γr)], where p is the frequency of allele A at both loci, r is the recombination rate between loci and γ is the per-gamete conversion rate. Somewhat unexpectedly, interchromosomal conversion (conversion between loci on different chromosomes) also generates positive disequilibrium, albeit very small. More interestingly, the behaviour of this disequilibrium as a function of recombination is unusual. If β is the interchromosomal conversion rate between a pair of loci, then = p(1 − p) [rβ/(β+r − βr)]. increases with increasing recombination, being zero for the case of complete linkage (r = 0), and maximized at r = 1/2. This unusual behaviour can be accounted for by the generation of excess coupling gametes when an interchromosomal conversion event is followed by recombination.

The pattern of inheritance of the autosomal recessive adrenal lipid depletion (ald) allele among the AKXL recombinant inbred mouse lines, derived from a cross between AKR/J and C57L/J, revealed that the ald locus is located on chromosome 1 near the dipeptidase-1 (Dip-1) locus. The linkage was confirmed in a backcross generation and the gene order ln-Dip-1-ald was established.

The Tcp-1 gene is located within the t complex and codes for a major testicular cell protein called p63/6.9. All wild-type chromosomes carry the Tcp-1b allele which codes for a basic form of this protein, while all complete t haplotypes carry the Tcp-1a allele which codes for an acidic form of this protein. It is not clear whether the Tcp-1 gene is associated with phenotypic effects of t haplotypes on embryogenesis and/or spermatogenesis, since the genetic basis for these effects is extremely complex. The elegant analysis of Lyon & Mason (1977) has allowed the identification and separation of a family of genetic factors which interact to produce the observed phenotypes associated with various combinations of t haplotypes. The data summarized in this report indicate that the Tcp-1a locus is separable from all of the identified t haplotype factors except for one; a complete correlation has been obtained between Tcp-1a and a proximal t haplotype factor which is involved in effects on transmission ratio distortion. Two other novel points emerge from this analysis. First, it appears that the tail interaction factor and the proximal sperm factors represent distinct genetic loci. Second, the accumulated data lead to the proposal that the TOrl chromosome carries a short segment of t haplotype chromatin containing Tcp-1a and proximal sperm factors involved in transmission ratio distortion and sterility.

Strains of Escherichia coli which contain the UV sensitivity gene exrA, repress UV induction of λ prophage in a novel manner. While exrA+ strains can be induced by UV irradiation and express this induction in a 2 h period, exrA mutants delay the expression of induction for almost 4 h, and the induction maximum is approximately 10% of the wild-type value. The kinetics of the delayed induction and superinfection experiments indicate that the lifting of immunity and induction of λ prophage occur simultaneously.